The present invention relates to a method for single-point probing of a workpiece by means of a sensor, in particular a tactile sensor, of a coordinate measuring machine. Furthermore, the present invention relates to a coordinate measuring machine for single-point probing of a workpiece.
Coordinate measuring machines are common in the prior art. A coordinate measuring machine is a machine comprising a measuring head which may be displaced relative to an object to be measured in a measurement volume. The measuring head is brought into a defined position relative to a measurement point at the object to be measured. The measurement point is probed in the case of tactile coordinate measuring machines, for example by means of a stylus arranged at the measuring head. Subsequently, spatial coordinates of the measurement point may be determined on the basis of the known position of the measuring head in the measurement volume. If the spatial coordinates of a plurality of defined measurement points are determined at an object to be measured, it is moreover possible to determine geometric dimensions or even the spatial form of the object to be measured. They serve for checking workpieces, for example as part of quality assurance, or for ascertaining the geometry of a workpiece completely as part of what is known as “reverse engineering.” Moreover, multifarious further application possibilities are conceivable.
In coordinate measuring machines of this type, different types of sensors may be used to capture the coordinates of a workpiece to be measured. By way of example, sensors that measure in tactile fashion are known in this respect, as are sold, for example, by the applicant under the name “VAST”, “VAST XT” or “VAST XXT.” Here, the surface of the workpiece to be measured is probed with a stylus, the coordinates of said stylus in the measurement space being known at all times. This probing may be carried out within the scope of single-point probing, in which each point to be measured on the workpiece is approached individually. For each single point, the stylus is brought into contact with the workpiece and subsequently lifted off the workpiece again. Such a stylus may be moved along the surface of a workpiece while it remains in contact with the workpiece, and so a multiplicity of measurement points may be captured at set time intervals during such a measuring process within the scope of a so-called “scanning method”.
It is moreover known to use optical sensors that facilitate contactless capturing of the coordinates of a workpiece. One example of such an optical sensor is the optical sensor sold by the applicant under the name “ViScan”.
The sensors may also be used in different types of mixed tactile and optical measuring constructions. Merely, one example of such a measurement construction is the product “O-INSPECT” by the applicant. In an appliance of this type, both an optical sensor and a tactile sensor are used to carry out various examination tasks on a machine and ideally with a single setup of a workpiece to be measured. In this way it is possible to easily carry out many examination tasks, for example in medical engineering, plastics technology, electronics and precision engineering. Various other constructions are, of course, also conceivable.
Conventionally, the sensor head is connected to a carrier structure or machine frame, which supports and moves the sensor system. The prior art has disclosed various carrier structures, for example portal systems, stand systems, horizontal arm systems and arm systems, all types of robotic systems and ultimately closed CT systems in the case of sensor systems operating with x-rays. Here, the carrier structures may moreover comprise system components which facilitate a positioning of the sensor head which is as flexible as possible. An example for this is the rotate-swivel-articulation from the applicant sold under the name “RDS”. Moreover, various adapters for connecting the various system components of the carrier structure among themselves and with the sensor system may be provided.
Coordinate measuring machines comprise a displaceable sensor head. In a coordinate measuring machine which underlies the following in an exemplary manner, the head is often fastened to the lower free end of a vertically arranged quill. The quill is displaceable such that the measuring head may be displaced perpendicular to a measuring table. The measuring table serves to receive an object to be measured. The quill in turn is arranged in a carriage on a crossbeam of a portal and it may be displaced in a first horizontal direction on the crossbeam by means of a carriage. Together with the quill, the portal may be displaced in a second horizontal direction, and so, overall, the measuring head may be displaced in three mutually perpendicular spatial directions. Here, the quill, the carriage and the portal form a machine frame. By way of example, the types of coordinate measuring machines sold by the applicant under the name “PRISMO”, “ACCURA” or “CenterMax” comprise examples of such machine frames. The maximum travels of the measurement head along the three movement directions determine a measurement space, within which spatial coordinates may be determined on an object to be measured.
In coordinate measuring machines which are used e.g. within the scope of quality assurance, the time duration or measurement time required to measure a workpiece is often an important parameter. A measurement time which is longer by even only fractions of a second may have a great disadvantage in the case of a multiplicity of workpieces to be examined. In this respect, attempts are made to carry out the measurements as quickly as possible or to keep the measurement time short. However, the speed of a measurement is often dependent on the accuracy of the measurement. Naturally, certain measurements are subject to accuracy requirements which should be strictly adhered to. As a rule, a measurement with a higher accuracy requires a longer period of time, and vice versa.
Hence, there have been deliberations in the prior art relating to bringing the time duration required for measuring a workpiece in line with the accuracy requirements.
Thus, documents DE 296 03 273 U1 and DE 295 04 239 U1 each disclose a coordinate measuring machine comprising a measuring contact probe for scanning the workpiece surface to be measured, motor-driven drives for the mobile measuring carriages of the coordinate measuring machine, by means of which the contact probe is guided, control electronics connected to the drives, a computer with monitor and input and output means for operating the coordinate measuring machine and for evaluating the measurement results, with the controller allowing different control parameters to be set, in particular allowing different measurement speeds of the drives to be set, wherein the input means are prepared for selecting different measuring problems and wherein the computer contains a memory in which control parameters appropriate to selectable measuring problems are stored.
Moreover, document WO 88/09915 A1 proposes a method for measuring nominally identical workpieces by means of a coordinate measuring machine. A workpiece should be measured at a low speed in order to facilitate measuring most of the workpieces at a high speed and to compensate the errors caused thereby. A series of error values for the differences between the slow measurement and the fast measurement is calculated for this workpiece. Subsequent fast measurements of successive workpieces are then corrected using these error values.
However, for single-point measurements by means of sensors, in particular by means of tactile sensors or else by means of optical sensors, so-called single-point probing, there still is a lack of reliable methods and coordinate measuring machines which carry out single-point measurements in an optimized manner, taking into account predetermined accuracy requirements, in particular in an optimized manner in respect of a time duration or measurement time that is as short as possible.
It is therefore an object of the present invention to remove the aforementioned disadvantages and provide a method and a coordinate measuring machine for single-point probing by means of sensors, in particular tactile sensors, which are able to handle different accuracy requirements.